In September 1843, antagonistic to the rule of King Otto I, Kallérgis, then a colonel, led a revolt that caused Otto to dismiss his Bavarian administration and to adopt a new, more liberal constitution (1844). Exiled in 1845, he returned in 1848 and tried unsuccessfully to instigate a revolution. In 1854 he was appointed minister in the cabinet of Aléxandros Mavrokordátos but was forced by the king to resign in October 1855. After the fall of Otto he took part in the negotiations leading to the accession of Prince William of Denmark to the Greek throne in 1863.